The spine consists of a series of bone structures termed “vertebrae.” Between each vertebra is a flexible, connective tissue termed an “intervertebral disc” which secures one vertebrae to another and functions as a shock absorber. Spinal fixation is a surgical technique in which one or more vertebrae are joined by an implant (e.g., a plate or rods.) to prevent relative movement of the spine, with the goal of live bone eventually fusing the adjacent vertebrae together.
Patients requiring spinal fusion typically suffer from either neurological deficits or severe pain which has not responded to conservative treatment. Typical conditions that are treated by spinal fusion procedure non-exclusively include: degenerative spinal conditions, discogenic pain, spinal tumor, vertebral fracture, scoliosis, kyphosis, spondylolisthesis, spondylosis, and other conditions that causes instability or pain in the spine.
Typically a spinal fixation procedure does not connect the patient's original vertebrae directly together; rather the intervertebral disc is usually completely or partially removed (disectomy) and/or one or more entire vertebral bodies are removed (corpectomy). The space remaining from the removed discs and vertebral bodies after a disectomy or corpectomy is typically replaced by a graft positioned between adjacent vertebrae to maintain proper length in the spinal column. After the surgery, it is desirable to have the living bone from the vertebrae span the inter-body graft thereby fusing the adjacent vertebrae together.
Traditionally, interbody grafts are fashioned from bone taken from a patient's skeleton, and are also referred to as “autografts.” As the harvesting of an autograft is painful for the patient, many surgeons now prefer the use of “allografts” which are harvested from a body other than the patient's. Interbody grafts may also be formed from synthetic materials such as titanium, carbon fiber and plastics. Unfortunately, grafts are associated with a relatively high rate of dislodgement due to the patient's neck movement during the healing process. To minimize the risk of dislodgement of the interbody graft posteriorly, toward the spinal cord, surgeons routinely mortise the graft by drilling a shelf into the vertebrae. To minimize the risk of dislodgement of the interbody graft anteriorly, surgeons routinely place a fusion plate across the inner space and secure it with screws extending into the vertebrae.
Placement of an anterior cervical plate with screw fixation is effective in preventing interbody graft dislodgement toward the esophagus and also enhances fusion by providing fixation between the vertebrae. In general, a cervical plate is attached to the anterior cortex of the vertebrae. This method can be disadvantageous as these plates are generally not low-profile, not easily compressible, nor easily centered. Cervical plates also tend to be long and may encroach upon the space of an adjacent disc space or vertebral body. Alternatively, other spinal devices are configured to be inserted directly into the disc space and attach to the vertebral endplates. This approach also is disadvantageous in that attachment to the vertebral endplates is not as strong as attachment to the hard anterior cortex of the vertebrae.
An additional problem of prior art plates is that their rigid attachment to the vertebral bodies can prevent compressive forces on the vertebral bodies, which in turn can prevent proper fusion of the graft. In attempt to counter this problem, new plate designs were developed that used slotted holes for the screws, configured such that the screws could slide in the slots, allowing compression of the graft. At least two problems are associated with these plates. Firstly, as the front of the spine compresses, compression in the back is limited by the facet joints posteriorly, which causes an abnormal forward angulation and flexion called kyphosis. Secondly, as the screws move down in the plate slots as the graft is compressed, the inferior and superior portions of the plate can encroach upon the neighboring disc areas or vertebral bodies, above and below the compressed vertebral bodies, and lead to advanced degeneration. In an attempt to counter the encroachment issue, new plates were designed that shorten when compressed. While addressing the encroachment problem, the shortening or translating plates can still lead to kyphosis as shown in FIG. 7. Stated otherwise, anterior vertebral plates that shorten in length when the graft and the surrounding vertebral bodies are compressed can still lead to the anterior area of the disc space collapsing more so than the posterior area.
Accordingly, there is a need in the art for improved spinal plates that are lower profile, more easily compressible and centered, do not encroach upon the space of a nearby disc or vertebral body, and that attach to bone that is stronger than the vertebral endplates. Additionally there is a need for anterior vertebral plates that prevent kyphosis, or forward angulation, of the spine and encourages the natural lordotic, or inward curvature, of cervical and lumbar regions of the vertebral column.